Every BlueStore block device has a single block label at the beginning of the
device. You can dump the contents of the label with:

ceph-bluestore-toolshow-label--dev*device*

The main device will have a lot of metadata, including information
that used to be stored in small files in the OSD data directory. The
auxilliary devices (db and wal) will only have the minimum required
fields (OSD UUID, size, device type, birth time).